Internal combustion engines fluidly couple to exhaust aftertreatment systems that purify exhaust gases generated as byproducts of combustion. Byproducts of combustion may include unburned hydrocarbons, carbon monoxide, nitrides of oxide, often referred to as NOx molecules, and particulate matter. Particulate matter may be generated in an internal combustion engine operating in a compression-ignition state. Exhaust aftertreatment systems may include oxidation catalysts, reduction catalysts, selective catalytic reduction catalysts and particulate filters.
Selective catalytic reduction catalysts may employ reductants for reducing NOx molecules to elemental nitrogen. The reductants may be stored on a surface or otherwise captured in selective catalytic reduction catalysts. Particulate filters capture particulate matter, and may require periodic regeneration. The process of regenerating a particulate filter may affect an amount of reductant that is stored on a selective catalytic reduction catalyst that is downstream of a particulate filter.